Ferrocenes have been employed for a very long time as combustion catalysts in propellant compositions. It has been found that their properties are improved by the addition of organosilyl radicals bound directly by carbon-silicon bonds to the cyclopentadienyl nucleus of the ferrocene. Such compounds are described, particularly in French Patent. Nos. 1,396,272, 1,396,273 and 2,567,890.
To synthesize these products several processes have already been described, in particular in the above mentioned French patents. The French patent FR 1,398, 235 describes a process for obtaining organosilylmetallocene by employing a reaction of acylation of metallocene using a silyl carboxylic acid halide, the carbonyl group being then reduced to a methylene radical using LiAlH4 as a reducing agent. This process makes it possible to synthesize the silylmetallocene but in low yields, approximately 20% of the acylation reaction.
The French Patent FR 2,567,890 describes a process for the manufacture of silylmetallocene compounds (e.g. dimethylsilyltetramethyleneferrocene) by hydrosilylation of metallocene compounds with H2PtCl6 catalyst and reduction of the resultant product to obtain silylmetallocene compounds.
The French Patent Nos. 2,667,318, 2,667,600 and 2,721,028 discloses the process for the synthesis of monohaloalkylferrocenes, an intermediate used in the synthesis of silylferrocene compounds. The French Patent FR 2,667,318 describes a process for the synthesis of monohaloalkylferrocenes by catalytic hydrogenation over PtO2 in acetic acid of monohaloalkanoylferrocenes. The crude synthetic product thus obtained, with purity generally in the region of 95%, requires subsequent purification in order to be used as an intermediate in the synthesis of ferrocene combustion catalysts for propellants. However, for this to be the case, it is necessary, as is shown in the Examples, to use a recrystallized, and thus very pure, starting haloalkanoylferrocene. Moreover, it turns out in practice that a purity of 95% is limiting for the above mentioned use and that it is preferable to use a haloalkylferrocene with a purity of 98%. Further, the platinum-based catalyst is fairly expensive and hydrogenation under pressure requires relatively expensive specific equipment.
The French Patent FR 2,667,600 describes the production, according to the abovementioned “Friedel-Crafts” method, of a crude synthetic haloalkanoylferrocene derivative, having purity generally in the region of 95%, by a combination of very precise operating conditions, in particular as regards the temperature, the concentration and the amount of the reactants. The crude product thus obtained is, however, insufficiently pure to be used directly, without purification, in the above mentioned process as described in French Patent FR 2,667,318. Such an use would lead, in fact, to a crude haloalkylferrocene derivative with a purity markedly less than 95% which cannot be used as is without prior purification in carrying out the subsequent stages.
The French Patent FR 2,667,600 certainly mentions that the addition of a cerous salt makes it possible to obtain a crude product with a purity greater than 95%, but this addition has virtually no influence on the content of 1,1′-di(haloalkanoyl)ferrocene, derivative, which remains excessively high.
The French Patent FR 2,721,028 describes a process for the synthesis of haloalkylferrocenes, comprising a first stage of reaction, in the presence of aluminum chloride as catalyst and inorganic solvent medium, of a haloalkyl carboxylic acid halide or haloalkylcarboxylic acid anhydride with a ferrocene derivative to produce an intermediate compound which is thereafter reduced with metal hydride to provide the haloalkylferrocene.
Thus, there exists a need for an improved process for preparation silylmetallocene compounds, such as for example 4-(dimethylsilyl)butylferrocene.